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.\" Syscall added in following commit
.\"	commit a2e2725541fad72416326798c2d7fa4dafb7d337
.\"	Author: Arnaldo Carvalho de Melo <acme@redhat.com>
.\"	Date:   Mon Oct 12 23:40:10 2009 -0700
.\"
.TH RECVMMSG 2 2016-10-08 "Linux" "Linux Programmer's Manual"
.SH NAME
recvmmsg \- receive multiple messages on a socket
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
.BI "#include <sys/socket.h>"

.BI "int recvmmsg(int " sockfd ", struct mmsghdr *" msgvec \
", unsigned int " vlen ","
.br
.BI "             unsigned int " flags ", struct timespec *" timeout ");"
.fi
.SH DESCRIPTION
The
.BR recvmmsg ()
system call is an extension of
.BR recvmsg (2)
that allows the caller to receive multiple messages from a socket
using a single system call.
(This has performance benefits for some applications.)
A further extension over
.BR recvmsg (2)
is support for a timeout on the receive operation.

The
.I sockfd
argument is the file descriptor of the socket to receive data from.

The
.I msgvec
argument is a pointer to an array of
.I mmsghdr
structures.
The size of this array is specified in
.IR vlen .

The
.I mmsghdr
structure is defined in
.I <sys/socket.h>
as:

.in +4n
.nf
struct mmsghdr {
    struct msghdr msg_hdr;  /* Message header */
    unsigned int  msg_len;  /* Number of received bytes for header */
};
.fi
.in
.PP
The
.I msg_hdr
field is a
.I msghdr
structure, as described in
.BR recvmsg (2).
The
.I msg_len
field is the number of bytes returned for the message in the entry.
This field has the same value as the return value of a single
.BR recvmsg (2)
on the header.

The
.I flags
argument contains flags ORed together.
The flags are the same as documented for
.BR recvmsg (2),
with the following addition:
.TP
.BR MSG_WAITFORONE " (since Linux 2.6.34)"
Turns on
.B MSG_DONTWAIT
after the first message has been received.
.PP
The
.I timeout
argument points to a
.I struct timespec
(see
.BR clock_gettime (2))
defining a timeout (seconds plus nanoseconds) for the receive operation
.RI ( "but see BUGS!" ).
(This interval will be rounded up to the system clock granularity,
and kernel scheduling delays mean that the blocking interval
may overrun by a small amount.)
If
.I timeout
is NULL, then the operation blocks indefinitely.

A blocking
.BR recvmmsg ()
call blocks until
.I vlen
messages have been received
or until the timeout expires.
A nonblocking call reads as many messages as are available
(up to the limit specified by
.IR vlen )
and returns immediately.

On return from
.BR recvmmsg (),
successive elements of
.IR msgvec
are updated to contain information about each received message:
.I msg_len
contains the size of the received message;
the subfields of
.I msg_hdr
are updated as described in
.BR recvmsg (2).
The return value of the call indicates the number of elements of
.I msgvec
that have been updated.
.SH RETURN VALUE
On success,
.BR recvmmsg ()
returns the number of messages received in
.IR msgvec ;
on error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
Errors are as for
.BR recvmsg (2).
In addition, the following error can occur:
.TP
.B EINVAL
.I timeout
is invalid.
.SH VERSIONS
The
.BR recvmmsg ()
system call was added in Linux 2.6.33.
Support in glibc was added in version 2.12.
.SH CONFORMING TO
.BR recvmmsg ()
is Linux-specific.
.SH BUGS
The
.I timeout
argument does not work as intended.
.\" FIXME . https://bugzilla.kernel.org/show_bug.cgi?id=75371
.\" http://thread.gmane.org/gmane.linux.man/5677
The timeout is checked only after the receipt of each datagram,
so that if up to
.I vlen\-1
datagrams are received before the timeout expires,
but then no further datagrams are received, the call will block forever.
.SH EXAMPLE
.PP
The following program uses
.BR recvmmsg ()
to receive multiple messages on a socket and stores
them in multiple buffers.
The call returns if all buffers are filled or if the
timeout specified has expired.

The following snippet periodically generates UDP datagrams
containing a random number:
.in +4n
.nf

.RB "$" " while true; do echo $RANDOM > /dev/udp/127.0.0.1/1234; "
.B      "      sleep 0.25; done"
.fi
.in

These datagrams are read by the example application, which
can give the following output:
.in +4n
.nf

.RB "$" " ./a.out"
5 messages received
1 11782
2 11345
3 304
4 13514
5 28421
.fi
.in
.SS Program source
\&
.nf
#define _GNU_SOURCE
#include <netinet/ip.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>

int
main(void)
{
#define VLEN 10
#define BUFSIZE 200
#define TIMEOUT 1
    int sockfd, retval, i;
    struct sockaddr_in addr;
    struct mmsghdr msgs[VLEN];
    struct iovec iovecs[VLEN];
    char bufs[VLEN][BUFSIZE+1];
    struct timespec timeout;

    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd == \-1) {
        perror("socket()");
        exit(EXIT_FAILURE);
    }

    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
    addr.sin_port = htons(1234);
    if (bind(sockfd, (struct sockaddr *) &addr, sizeof(addr)) == \-1) {
        perror("bind()");
        exit(EXIT_FAILURE);
    }

    memset(msgs, 0, sizeof(msgs));
    for (i = 0; i < VLEN; i++) {
        iovecs[i].iov_base         = bufs[i];
        iovecs[i].iov_len          = BUFSIZE;
        msgs[i].msg_hdr.msg_iov    = &iovecs[i];
        msgs[i].msg_hdr.msg_iovlen = 1;
    }

    timeout.tv_sec = TIMEOUT;
    timeout.tv_nsec = 0;

    retval = recvmmsg(sockfd, msgs, VLEN, 0, &timeout);
    if (retval == \-1) {
        perror("recvmmsg()");
        exit(EXIT_FAILURE);
    }

    printf("%d messages received\\n", retval);
    for (i = 0; i < retval; i++) {
        bufs[i][msgs[i].msg_len] = 0;
        printf("%d %s", i+1, bufs[i]);
    }
    exit(EXIT_SUCCESS);
}
.fi
.SH SEE ALSO
.BR clock_gettime (2),
.BR recvmsg (2),
.BR sendmmsg (2),
.BR sendmsg (2),
.BR socket (2),
.BR socket (7)
